1. Field of the Invention
The present invention relates to an X-ray apparatus, and more particularly, to an X-ray apparatus provided with an X-ray tube having slide bearings therein and using a liquid metal as a lubricant.
2. Description of the Related Art
Many of X ray apparatuses, such as X-ray photographing apparatuses, X-ray exposure apparatuses, CT scanners, etc. incorporate a rotating-anode X-ray tube for use as an X-ray radiation source. In the X-ray tube of this type, as is generally known, a disk-shaped anode target is fixed to a rotating structure, which is rotatably supported on a stationary structure by a bearing section formed therebetween. A magnetic stator coil is arranged outside a vacuum envelope, and the rotating structure is rotated at high speed when the stator coil is energized. While the rotating structure is rotating at high speed, an electron beam emitted from a cathode is applied to the anode target, whereby X-rays are irradiated. The bearing section is composed of bearings, such as ball bearings, or dynamic-pressure slide bearings which have spiral grooves on their bearing surfaces. The slide bearings use a liquid metal lubricant which, formed of gallium or a gallium-indium-tin (Ga-In-Sn) alloy, becomes liquid during operation. X-ray tubes which use the slide bearings are disclosed in, for example, Published Japanese Patent Application No. 60-21463 and Published Unexamined Japanese Patent Applications Nos. 60-97536, 60-117531, 60-160552, 62-287555, 2-227947, and 2-227948.
The liquid metal lubricant applied in the slide bearings of these rotating-anode X-ray tubes has a practical melting point of about 10.degree. C. at the lowest. More specifically, the melting point of a low-melting Ga-In-Sn alloy is 10.7.degree. C., and that of a low-melting Bi-In-Pb-Sn alloy, whose bismuth (Bi) content is relatively high, is 57.degree. C. In some cases, the X-ray tube apparatuses are used at a temperature lower than the melting point of the metal lubricant, so that the lubricant in the bearing section of the X-ray tube is frozen or solidified before the operation of the apparatus is started. In this state, the anode target cannot rotate so that the anode target surface is damaged by an excessive temperature rise due to the electron bombardment. It is necessary, therefore, to melt and liquidify the lubricant by heating the bearing section to a temperature higher than the melting point of the lubricant before starting the rotation of the anode target of the X-ray tube. To attain this, the X-ray tube disclosed in Published Unexamined Japanese Patent Application No. 60-160552 is arranged so that a heat source, such as a heating coil, heat radiator, or high-frequency radiator, is provided in- or outside the X-ray tube, and that heat radiation of a cathode filament is utilized.
Inevitably, however, this arrangement requires the additional use of an extra heat source. Further, it takes a lot of time to increase the temperature of the bearing section by utilizing the heat radiation of the cathode filament.